A study of efficiency in low resistivity silicon solar cells

A general device analysis program has been utilized to study the efficiency of silicon solar cells. The analysis is applied to specific geometries of both n⁺-p and n⁺-p-p⁺ solar cells and involves a numerical solution of the basic transport and continuity equations. This approach allows solutions free of typical limiting assumptions involved in solving the device equations apart from those relating to lifetime, mobility variations and diffused region profiles. The analysis includes available empirical information of diffusion length, mobility, and lifetime as a function of doping as well as a Gaussian profile for the diffused region. Results are presented which illustrate the limitations of efficiency as a function of doping. It was found that the maximum efficiencies for both types of cell converge at lower resistivities to around 16% with AMO radiation and a single layer absorbing SiO antireflecting film. It was also found that the minority carrier lifetime, both in the n⁺ surface and p-type bulkk regions, presents serious limitations to conversion efficiency particularly in the low resistivity cells.     

Quadratic recombination in silicon and its influence on the bulk lifetime

The coefficient of nonradiative excitonic recombination by the Auger mechanism involving deep-level centers in n-Si was determined by comparing the theoretical dependence of the effective bulk lifetime on the doping level with the experimental dependence. It is shown that this mechanism controls the bulk lifetime in silicon at doping levels on the order of or above 10¹⁶ cm^?3. This mechanism is more pronounced at shorter bulk lifetimes τ _v0 and low doping levels. The dependences of the bipolar diffusion length in n-Si on the doping level (using the parameter τ _v0) were calculated.     

Minority carrier lifetimes in silicon solar cells determined from spectral and transient measurements

Measurements of the spectral collection efficiency and short circuit current decay rate following an X-ray pulse have been made on three types of single crystal silicon solar cells. The cell types were n⁺ - p, p⁺ - n, and p⁺ - n - n⁺ with base resistivities of 0.3, 10 and 10 Ω-cm, respectively. Minority carrier lifetimes were determined from both experiments using analytical or device code calculations, as required. For the n⁺ - p and p⁺ - n cells, nominal lifetimes of 2 and 5 ωsec, respectively, were obtained. A lifetime greater than 100 ωsec was inferred for the p⁺ - n - n⁺ device. This value represents a minimum estimate since our analysis is inaccurate when the diffusion length exceeds the cell thickness, as is the case here. The difference in base lifetime for the p⁺ - n and p⁺ - n - n⁺ structures is attributed to gettering during the phosphorus diffusion to form the back surface field layer.     

Diffusion-profile measurement in InP with Schottky diodes

Measurements of the doping profile resulting from the diffusion of Cd into lowly-doped n-type InP are reported. The measurements were taken with Au Schottky contacts. In order to probe the doping profile in its entirety with the limited resolution depth of the Schottky diodes, thin layers of the diffused samples were removed by chemical etching in a well-controlled fashion. The etching procedure leading to smooth crystal surfaces is described in detail. The diffusion profile of Cd is characterized by a flat portion in the low 10¹⁸ cm^?3 extending to a sharp gradient coinciding with the shallow diffusion front detectable by cleaving and stain etching. Between the shallow and second diffusion front very low p-type doping is found whereas beyond the second diffusion front the n-type conductivity of the substrate is attained. Thus Cd leads to a p⁺pn transition in n^?-InP.     

Voltage dependence of the differential capacitance of a p +-n junction

The dependences of the differential capacitance and current of a p ⁺-n junction with a uniformly doped n region on the voltage in the junction region are calculated. The p ⁺-n junction capacitance controls the charge change in the junction region taking into account a change in the electric field of the quasi-neutral n region and a change in its bipolar drift mobility with increasing excess charge-carrier concentration. It is shown that the change in the sign of the p ⁺-n junction capacitance with increasing injection level is caused by a decrease in the bipolar drift mobility as the electron-hole pair concentration in the n region increases. It is shown that the p ⁺-n junction capacitance decreases with increasing reverse voltage and tends to a constant positive value.     

Passivation of defects in polycrystalline silicon solar cells by molecular hydrogen annealing

The paper presents a new technique for passivating grain boundaries and intragrain defects in the polycrystalline silicon material of the solar cells. Enhancement in the minority-carrier diffusion length has been achieved by thermal annealing in molecular hydrogen in previously formed n⁺ p junctions. The annealing temperature dependence of the diffusion length has been studied in the temperature range of 400 to 700° C. The study shows a significant increase in the diffusion length from 21-8/ im to 67-8/ jm by this process. The diffusion length increases with annealing in molecular hydrogen and shows a peak value at 600° C.     

Minority-carrier lifetime in dielectrically isolated single-crystal silicon films defined by electrochemical etching

The minority-carrier lifetime has been measured in thin, dielectrically isolated single-crystal silicon films defined by electrochemical etching. Both transient-current measurements on deep-depletion MOS transistors and recombination-current measurements on bipolar junction transistors have been use to determine the lifetime. Values of the order of 1 μ sec have been observed in both n-and p-type films with dopant concentrations of about 10¹⁵ cm^?3. It was found that the characteristics of the MOS transistors were not dominated by generation at either surface of the thin film. No differences were seen between the characteristics of bipolar transistors fabricated in the thin films and those of transistors fabricated in bulk control wafers.     

A calculation of the capacitance-voltage characteristics of p+-InP/n-InP/n-InGaAsP photodiodes

An analytic solution for the capacitance-voltage characteristics of p⁺?InP/n?InP/n?InGaAsP diodes has been obtained. The solution, which closely approximates that obtained using numerical methods, indicates that the dip followed by a peak generally observed in the effective carrier concentration profile as derived from experimental capacitance measurements is likely due to the properties of the n-n heterojunction rather than actual variations in the doping concentration. The availability of the analytic solution greatly facilitates the study of the effects of various parameters on these characteristics. The influence of grading and the collection of holes in the notch formed by the valence band discontinuity is also briefly considered.     

The open-circuit voltage of back-surface-field (BSF) p-n junction solar cells in concentrated sunlight

Simple analytical expressions for the open-circuit voltage of the n⁺?p?p⁺ and p⁺?n?n⁺ BSF solar cells, which are valid for both the low- and high-levels of optical illumination, are derived. Based on the principle of superposition the open-circuit voltage of both the n⁺?p?p⁺ and p⁺?n?n⁺ solar cells are expressed in terms of the short-circuit current and the known saturated dark current. Effects of the high-low junction doping, the energy-gap shrinkage, and the dimensions of the BSF solar cells on the open-circuit voltage are included. The numerical results of the derived expressions are found to be in good agreement with the exact numerical analysis of Fossum et al. The optimal design considerations based on the known characteristics of the open-circuit voltage are also discussed.     

Numerical analysis on abnormal thyristor forward voltage increase due to heavy doping in gated p-base layer

An abnormal forward voltage increase was observed for a p-base gated double diffused n⁺pn^?p⁺ high power thyristor with high impurity concentration at the n⁺-p emitter-base junction. Accurate numerical analysis shows that heavy doping effects are the most responsible mechanism for the abnormality and that depletion layer formation at the center junction accompanies it.It will be shown that appropriate control of the impurity concentration at the emitter-base junction is necessary to avoid this abnormality by realizing the common base transistor current gain of greater than 0.73 for n⁺n^?-portion.     

Influence of the electron-hole equilibrium shift on transition-metal diffusion in GaAs

Diffusion of impurities of transition metals Fe, Cu, and Cr in heavily doped p ⁺-, n ⁺-, and intrinsic (at diffusion temperature) GaAs is studied. A technique in which impurity diffuses into GaAs-based structures with heavily doped layers (p ⁺-n or n ⁺-n) was used. It is shown that the impurity diffusivity values in p ⁺-GaAs and n ⁺-GaAs are significantly higher and lower, respectively, than for i-GaAs. The results obtained are discussed taking into account the effect of the electron-hole equilibrium shift in semiconductors on the diffusion of impurities migrating according to the dissociative mechanism. The interstitial-component concentration for Fe, Cu, and Cr impurities in GaAs was determined at the diffusion temperature.     

CdHgTe heterostructures for new-generation IR photodetectors operating at elevated temperatures

The parameters of multilayer Cd_xHg_1–xTe heterostructures for photodetectors operating at wavelengths of up to 5 μm, grown by molecular-beam epitaxy (MBE) on silicon substrates, are studied. The passivating properties of thin CdTe layers on the surface of these structures are analyzed by measuring the C–V characteristics. The temperature dependences of the minority carrier lifetime in the photoabsorption layer after growth and thermal annealing are investigated. Samples of p ⁺–n-type photodiodes are fabricated by the implantation of arsenic ions into n-type layers, doped with In to a concentration of (1–5) × 10¹⁵ cm^–3. The temperature dependences of the reverse currents are measured at several bias voltages; these currents turn out to be almost two orders of magnitude lower than those for n ⁺–p-type diodes.     

High-efficiency GaSb photocells

High-current solar cells based on gallium antimonide and intended for use in solar modules and systems with solar-spectrum splitting at large solar light concentration ratios, in thermophotovoltaic generators with a high-temperature emitter, and in laser energy converters have been designed and fabricated by the diffusion of zinc from the gas phase. The influence exerted by the thickness of the p ⁺ diffusion layer on the basic characteristics of the solar cell has been studied. The optimal doping profile and the p-n-junction depth providing a high photovoltaic conversion efficiency at photocurrent densities of up to 100 A cm^?2 have been determined.     

Characterization of <Emphasis Type="Italic">n</Emphasis>-Type and <Emphasis Type="Italic">p</Emphasis>-Type Long-Wave InAs/InAsSb Superlattices

The influence of dopant concentration on both in-plane mobility and minority carrier lifetime in long-wave infrared InAs/InAsSb superlattices (SLs) was investigated. Unintentially doped (n-type) and various concentrations of Be-doped (p-type) SLs were characterized using variable-field Hall and photoconductive decay techniques. Minority carrier lifetimes in p-type InAs/InAsSb SLs are observed to decrease with increasing carrier concentration, with the longest lifetime at 77 K determined to be 437 ns, corresponding to a measured carrier concentration of p ₀ = 4.1 × 10¹⁵ cm^?3. Variable-field Hall technique enabled the extraction of in-plane hole, electron, and surface electron transport properties as a function of temperature. In-plane hole mobility is not observed to change with doping level and increases with reducing temperature, reaching a maximum at the lowest temperature measured of 30 K. An activation energy of the Be-dopant is determined to be 3.5 meV from Arrhenius analysis of hole concentration. Minority carrier electrons populations are suppressed at the highest Be-doping levels, but mobility and concentration values are resolved in lower-doped samples. An average surface electron conductivity of 3.54 × 10^?4 S at 30 K is determined from the analysis of p-type samples. Effects of passivation treatments on surface conductivity will be presented.     

Dependence of the electrical parameters of MBE-grown Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Te films on the level of doping with indium

Dependences of the minority-carrier lifetime and electron mobility in Cd _x Hg_{1 ? x} Te films on their indium-doping level are studied. Films with x ≈ 0.22 grown by molecular-beam epitaxy on GaAs substrates were in situ doped with indium across their entire thickness. The temperature dependences of the lifetime were studied in the temperature range 77–300 K. The decrease in the lifetime, observed as the doping level increases, is governed by the mechanism of Auger recombination. As the doping level becomes higher, the mobility decreases in qualitative agreement with theoretical calculations.     

Small-signal analysis of punch-through injection microwave devices

A theoretical small-signal analysis of punch-through injection microwave devices is given. A numerical study of a silicon p⁺?n?p⁺ structure is performed, which shows good agreement with experimental measurements by Snapp and Weissglas for a diode with a doping density of 1·2 × 10¹⁵/cm³. Negative resistance is also calculated for diodes with doping densities of 0·6 × 10¹⁵/cm³ and 5 × 10¹⁵/cm³. A partially analytical mode, including the lowfield region, is developed and compared with the numerical calculation. Ohmic losses for devices with low impurity concentrations and diffusion for devices with high impurity concentrations are shown to be significant factors.The noise spectrum is calculated numerically from the assumption of two noise sources, injection noise and diffusion noise. The noise measure is determined and shown to be in good agreement with experiments by Björkman and Snapp.     

Optimization of n+ self-aligned short-channel normally-off GaAs MESFETs by two-dimensional numerical device simulation

The behaviour of n⁺ self-aligned short-channel normally-off GaAs MESFETs with increasing channel doping concentration is investigated by detailed computer simulation. Characteristic data of n⁺ self-aligned technology like n⁺-gate spacing and projected range of n⁺-implant are optimized. For the calculation a two-dimensional numerical GaAs device simulation program developed at Dresden University of Technology is used. Increasing channel doping concentration up to 10¹⁸ cm⁻³ decreases sensitivity of threshold voltage upon gate length by about 25–30%. Furthermore, transconductance and K-value are increased by more than twofold. A transconductance of 610 mS/mm was calculated for a GaAs normally-off MESFET with 0.5 μm gate. This is a remarkably high value for MESFETs.     

Empirical Study of the Disparity in Radiation Tolerance of the Minority-Carrier Lifetime Between II–VI and III–V MWIR Detector Technologies for Space Applications

The degradation of the minority-carrier recombination lifetime of various III–V nBn and II–VI HgCdTe midwave-infrared space detector materials under stepwise 63-MeV proton irradiation up to fluence of 7.5 × 10¹¹ cm^?2 and above has been measured using time-resolved photoluminescence while samples were held at 120 K to limit thermal annealing. As expected, the recombination rate of each sample was found to increase with proton fluence at a nearly constant rate, implying a near-linear increase in defect concentration. The rate of change of the carrier recombination rate, herein called the minority-carrier lifetime damage factor, was then plotted as a function of the initial recombination rate for each sample. Juxtaposing the III–V and II–VI results revealed a distinct disparity, with the incumbent detector material HgCdTe being roughly an order of magnitude more radiation tolerant to displacement damage from proton irradiation than any of the nBn materials. The results for the latter also suggest some degree of interrelation between the damage factor and initial lifetime. The behavior of the lifetime of each material under annealing revealed that HgCdTe exhibited nearly 100% recovery at 295 K whereas III–V materials recovered to only about 50% under the same conditions.     

A new method for computer-aided optimization of solar cell structures

A new method for truly optimizing the design of intrinsic solar-cell structures has been established. To support the proposed method, a device modeling technique specialized for n⁺p solar cells with uniform doping has been developed, and the constrained optimization technique extended from Rosenbrock's algorithm has been implemented. All parameters to be optimized are simultaneously adjusted in a systematic manner, resulting in a truly optimum design. Several design examples for n⁺?p solar cells, such as maximizing cell efficiency and minimizing cell thickness while maintaining the acceptable characteristics, have been used to demonstrate the validity of the proposed method. Moreover, this method has also been successfully applied to the optimum design of other solar cell structures.     

Properties of Mn-doped p-type InxGa1-xAsyP1-y grown by liquid-phase epitaxy

Epitaxial layers of p-type In_xGa_1-xAs_yP_1-y doped with Mn were grown by liquid-phase epitaxy on (111)-B oriented InP substrates at a growth temperature of 635°C. The doping characteristics and electrical and luminescent properties were studied and compared with those in Zn-doped epilayers. The distribution coefficient of Mn was about 0.1–0.3. The p-type epilayers with hole concentration of up to 3 × 10¹⁸ cm^?3 could be easily obtained by Mn doping. The activation energy of the Mn acceptor was about 40 meV. Mn doping yielded a broad photoluminescent emission spectrum which probably arises from d-shell interaction of the Mn as well as strong phonon coupling. From electron beam-induced current measurements in p-n heterojunctions utilizing Mn, a value of L_n = 2 μm was obtained for the minority carrier diffusion length of electrons in the p-type region.